Plasma processing device for surfaces

ABSTRACT

A mechanism for plasma surface treatment includes a rotating head having at least one eccentrically disposed plasma nozzle for generating a plasma jet directed in parallel with the axis of rotation. The nozzle includes a swirl system for swirling the plasma jet.

BACKGROUND OF THE INVENTION

The invention relates to an apparatus for plasma treatment of surfaces.More particularly, the invention relates to a pretreatment enabling thecoating of surfaces of synthetic resin materials with adhesives,printing inks and the like. Such a pretreatment is necessary because thesurfaces of synthetic resins can normally not be wetted with liquids anddo therefore not accept the printing ink or the adhesive. Thepretreatment changes the surface structure of the synthetic resin sothat it becomes wettable with liquids having a comparatively largesurface tension. The surface tension of the liquids with which thesurface can still be wetted is an indictor for the quality of thepretreatment.

A well established method for the pretreatment of synthetic resinsurfaces is based on the principle of corona discharge. In this methodthe resin to be treated is typically passed through between twoelectrodes which are coated with a ceramic material and to which a highvoltage with high frequency is applied so that a corona discharge occursthrough the synthetic resin material. However, this method is suitableonly for relatively thin workpieces having a flat surface, such asplastic foils.

DE 195 32 112 A discloses an apparatus for pretreatment of surfaces bymeans of a plasma jet. Thanks to a specific construction of the plasmanozzle a relatively cool but nevertheless highly reactive plasma jet isachieved which has a shape and dimensions similar to those of a flame ofa candle and, as a consequence, permits also the pretreatment ofprofiles having relatively deep recesses. Due to the high reactivity ofthe plasma jet a short pretreatment time is sufficient, so that theworkpiece can be passed along the plasma jet with a relatively highvelocity. Thus, as a result of the comparatively low temperature of theplasma jet, a pretreatment of heat-sensitive plastic materials is alsopossible. Since no counter electrode is necessary on the back side ofthe workpiece, it is also possible to pretreat the surfaces ofarbitrarily thick block-like workpieces, hollow bodies, and the like.For a uniform pretreatment of larger surfaces, the cited publicationdiscloses an array of a plurality of staggered plasma nozzles. In thiscase, however, relatively high expenses are necessary for the equipment.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention to provide an apparatus for plasmasurface treatment which permits a rapid and efficient pretreatment oflarger surfaces at low equipment expenses. This object is achieved by anapparatus which comprises a rotating head which has at least oneexcentrically disposed plasma nozzle for generating a plasma jetdirected in parallel with the axis of rotation.

When the workpiece is moved relative to the rotating head which rotateswith a high speed of revolution, the plasma jet sweeps a stripe-shapedsurface zone of the workpiece, and the width of this zone corresponds tothe diameter of the circle described by the revolving plasma nozzle.Thus, a comparatively large surface can be pretreated in a rational way.

In this context, it is tolerable that the intensity of the pretreatmentin the stripe being swept is not completely uniform. The parameters ofthe pretreatment, especially the speed of revolution of the rotatinghead and the translational speed of the workpiece relative to therotating head can, within broad ranges, always be selected such that asufficient wettability of the workpiece is achieved everywhere in thepretreated stripe and, on the other hand, the material is not thermallydamaged in the zones which have been treated most intensively.

Thanks to the long range of the plasma jet, the pretreated surface ofthe workpiece can also be curved or profiled. Thus, the apparatus isalso suitable for example for the pretreatment of frame profiles forwindows or doors made of plastics, plastic bottles or buckets, and thelike.

It has been found to be particularly useful in the apparatus accordingto the invention that the rotation of the head and the correspondingrotation of the plasma jet creates an extended vortex and, as a result,a low pressure is formed in the center of the vortex. This low pressurecounteracts a radial escape of the plasma jet and has the effect thatthe plasma jet is “sucked” to the surface of the workpiece, so that itcomes into intimate contact with the surface of the workpiece.

Preferably, two or more plasma jets are mounted to the rotating headwith uniform angular spacings, so that at least two plasma jets areoperating and the treatment time is shortened correspondingly. Thisarrangement has also the advantage that the rotating head issubstantially balanced due to the symmetric arrangement of the plasmanozzles.

Thanks to the high speed of revolution of the rotating head, which mayamount to, for example, 1000 min⁻¹ or more, Coriolis forces and the likelead to a swirl in the plasma jets exiting from the individual plasmanozzles. In addition, each plasma nozzle preferably has its own swirlarrangement which assures a stabilisation and focusing of the plasmajet. The sense of rotation of the rotating head should in this case beadapted to the sense of the swirling movement in the individual plasmanozzles.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be explained in conjunction withthe drawings, in which:

FIG. 1 is an axial section of a rotating head;

FIG. 2 is a front view of the rotating head; and

FIG. 3 is an axial section through the whole apparatus corresponding tothe plane III—III in FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

FIG. 1 shows a rotating head 10 which rotates about its central axis,which is the vertical axis in FIG. 1, and is surrounded by a stationarycylinder 12 serving as a protecting shield. The rotating head 10 has twodiametrically opposed plasma nozzles 14 mounted to an annulardistributor block 16 and arranged to emit plasma jets 18 in a directionin parallel with the axis of rotation. When the head 10 is movedrelative to the surface of a workpiece 20 in a direction normal to theplane of the drawing in FIG. 1 and rotates with a high speed ofrevolution, the plasma jets 18 sweep relatively uniformly over a stripeon the surface of the workpiece having a width W of, for example, 8 cm.

The mouths 22 of the plasma nozzles are disposed in a common plane in aface plate 24 which is held co-rotatably at the distributor block 16 bytwo bars 26. The bars 26 are disposed in a plane normal to the plane ofthe plasma nozzles 14 and are connected with one another at the faceplate 24 by a cross piece 28.

Each plasma nozzle 14 has an essentially cylindrical metal casing 30tapered towards the mouth 22 and forming a vortex channel 32 conicallytapered towards the mouth 22. The mouth 22 of the plasma nozzle is againsignificantly restrained in comparison to the internal cross section ofthe vortex channel 32. The upstream end of the casing 30 is rigidlyconnected to an adapter 34 made of metal and embedded in the distributorblock 16. A ceramic tube 36 is coaxially disposed in the adapter 34 andthe end of the casing 30 adjacent thereto and accommodates a swirl ring38 which is eclectically insulated from the adapter 34 and the casing30. The swirl ring 38 forms an electrode pin 40 which projects into thevortex channel 32 and is surrounded by a crest of swirl orifices 42. Theadapters 34 and the distributor block 16 are formed with gas passages 44for supplying working gas to the plasma nozzles. The adapters 34 and thedistributor block 16 further comprise cable passages 46 for high voltagecables which have not been shown and by which a voltage is applied tothe swirl rinds 36 and the electrode pins 40 respectively connectedtherewith.

When the apparatus is in operation, pressurised air is supplied as aworking gas via the gas passages 44. The pressurised air passes throughthe swirl orifices 42 of the swirl ring 38 and is swirled thereby sothat it flows through the vortage channel 32 to the mouth 22 of theplasma nozzle in a vortex fashion. An AC voltage of several kV having afrequency of, for example, 20 kHz is applied to the electrode pin 40,whereas the casing 30 of the plasma nozzle is grounded via thedistributor ring 16. When the voltage is switched on, the high frequencyat first creates a corona discharge between the swirl ring 38 and theceramic tube 36 serving as a dielectricum. This corona discharge thenignites an arc discharge between the electrode pin 40 and the casing 30.The electric arc will however not pass radially from the electrode pin40 to the surrounding wall of the casing 30, but is instead entrained bythe swirling gas flow and is channelled in the core of the gas vortex,so that it passes from the electrode pin 40 straight along the centralaxis of the vortex channel 32 to the mouth 22, and it is only then thatit is radially branched towards the edge of the mouth. This has thedesired result that a highly reactive and well focused plasma jet isformed which nevertheless is relatively cool and, in addition, thanks toits swirling motion, will excellently mate with the surface of theworkpiece 20.

FIG. 2 shows the arrangement of the gas passages 44 in the distributorblock 16. The gas passages are formed by a system of bores crossing oneanother and respectively closed by plugs 48 at their outer ends and eachconnecting an axial inlet passage 50 with an associated one of theplasma nozzles 14.

As is shown in FIG. 3, the distributor block 16 is mounted to athickened end of a shaft 52 which is rotatably supported in a bearinghousing 54. The bearing housing 54 has an inlet port 56 for compressedair leading to a pressure chamber 58 through which the shaft 52 passes.The pressure chamber 58 is hermetically sealed by a pressure-tightconstruction of the bearings for the shaft 52. The shaft 52 has twoexcentric, axial gas passages 60 connected at one end to the pressurechamber 58 and at the other end to the above-mentioned axial passages 50of the distributor block 16. Thus, pressurised air is supplied to theplasma nozzles 14 of the rotating head 10 via the inlet port 56.

Inside of the pressure chamber 58 the shaft 52 carries a collector ring62 which is in sliding engagement with a wiper contact 64. This wipercontact assures the grounding of the shaft 52 and the distributor block16 as well as the casings of the plasma nozzles 14 connected thereto.

A connector housing 66 is adjoined to the bearing housing 54 on the sideopposite to the rotating head 10 and accommodates two insulated wipercontacts 68 for the high voltage supply of the plasma nozzles 14. Anextension of the shaft 52 projecting into the connector housing 66carries an insulator 70 on which two collector rings 72 are disposedwhich are each in engagement with one of the wiper contacts 68. Thecollector rings 72 are electrically insulating from one another and fromthe shaft 52 and the connector housing 66 by the insulator 70 which isformed with radially projecting disks 74. In addition, the insulator 70has two axial cable passages 76 which each pass from one of thecollector rings 72 to the end face of the insulator 70 engaging ashoulder of the shaft 52. Recesses 78 formed in this shoulder connectthe cable passages 76 to a center bore 80 of the shaft 52. The oppositeend of this center bore 80 is connected by radial recesses 82 to thecable passages 76 of the distributor block 16 shown in FIG. 1. Thus, itis possible to electrically connect the collector rings 72 with theswirl rings 38 and electrode pins 40 of the plasma nozzles 14 by meansof the high voltage cables mentioned above. The separate electric supplylines for the two plasma nozzles make it possible to supply to eachplasma nozzle a high voltage from a separate high voltage source. Thisis essential for being able to supply the necessary ignition voltage forigniting one of the plasma nozzles even when an arc discharge is alreadyoccurring in the other plasma nozzle.

What is claimed is:
 1. Apparatus for plasma surface treatment,comprising a rotatable head having at least one eccentrically disposedplasma nozzle for generating a plasma jet directed in parallel with theaxis of rotation.
 2. Apparatus according to claim 1, wherein a pluralityof the plasma nozzles are arranged at even angular spacings around theaxis of rotation.
 3. Apparatus according to claim 2, wherein each plasmanozzle has a swirl system for swirling the plasma jet.
 4. Apparatusaccording to claim 3 wherein each plasma nozzle has an elongated casingdefining a grounded electrode and forming an elongated vortex channelaccommodating the swirl system and including a mouth through which anelectric arc exits the nozzle, the channel being tapered toward themouth, and wherein each plasma nozzle further has a high voltageelectrode disposed in the channel, the swirl system and the vortexchannel arranged such that an electric arc discharge from the highvoltage electrode is channeled in a vortex core extending along acentral axis of the vortex channel and branched to the casing only whenreaching the mouth.
 5. Apparatus according to claim 1 wherein eachplasma nozzle has a swirl system for swirling the plasma jet. 6.Apparatus according to claim 1 wherein the plasma nozzle is powered by ahigh frequency voltage.
 7. Apparatus according to claim 6, wherein theplasma nozzle has a dielectric member separating the grounded electrodeand the high voltage electrode from one another and permitting a coronadischarge serving as an ignition discharge.
 8. Apparatus according toclaim 1, wherein the plasma nozzle is mounted to a distributor blockhaving gas passages and cable passages for respectively supplying aworking gas and an operating voltage to the plasma nozzle, the blockmounted to one end of a shaft which passes through a pressure chamberinto which the working gas is supplied, and which also passes through aconnector housing and is provided with collector rings inside of theconnector housing, the shaft having axial gas and cable passages forconnection with corresponding passages in the distributor block. 9.Apparatus according to claim 8, wherein the shaft is journaled in abearing housing forming the pressure chamber and freely projects intothe contact housing disposed on a side of the bearing housing disposedopposite to the rotating head.